1. Field of the Invention
This invention relates to novel 3,4-pyridine-dithiol compounds, and in particular to pyridinethiol compounds which are useful as intermediates for producing pyridinethiolate nickel complexes used as light stabilizers and near-infrared-rays absorbing materials, and useful as chelating agents to obtain dithiolate metal complexes, and a method of producing the 3,4-pyridine-dithiol compounds.
2. Discussion of Background
As the recent development of infrared filters, optical recording media, liquid crystal display devices, laser printers and electrophotographic copying apparatus, the development of a dye which is capable of efficiently absorbing the light in the region of infrared to near infrared is desired.
As the above-mentioned dye which is capable of absorbing the near infrared rays, squarylium dyes, cyanine dyes, metal phthalocyanine dyes and platinum bis(dithio-.alpha.-diketone) complex compounds are conventionally known.
The squarylium dyes have the shortcomings that the absorbance thereof is low and the film-forming properties thereof are unsatisfactory.
The cyanine dyes become chemically unstable when their molecular chain is lengthened to increase the absorbance in the near infrared region.
The metal phthalocyanine dyes cannot be worked into a thin-film by the solution coating method because of their low solubility in organic solvents. This will considerably restrict the application thereof. In addition, an amorphous film of the metal phthalocyanine dye is readily crystallized.
Platinum bis(dithio-.alpha.-diketone) complex compounds have poor film-forming properties and are chemically unstable.
Various benzenethiolate metal complexes are disclosed in Japanese Laid-Open Patent Applications 63-288786, 63-209890 and 63-288785, which serve as light stabilizers in an organic-dye-type optical disk.
However, the above-mentioned benzenethiolate metal complexes cannot be easily coated on a substrate made of a resin with poor resistance to non-aqueous solvents, such as polycarbonate. This is because the benzenethiolate metal complexes are soluble in the non-aqueous solvents, but are not soluble in aqueous solvents such as alcohol-based solvents.
Furthermore, when an optical disk is prepared by using the benzenethiolate metal complexes, the bit error ratio is increased during the repeated operations, which will be accompanied by the decrease in reliability.
As previously described, most of the conventional materials serving as light stabilizers and near-infrared-rays absorbing materials do not satisfy all the requirements at the same time, such as high solubility in organic solvents, high compatibility with resins, good film-forming properties and chemical stability.
It is recently confirmed that pyridinethiolate nickel complexes effectively function as the light stabilizer and near-infrared-rays absorbing material, with all the aforementioned requirements being satisfied.